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Most of the food we eat is complex carbohydrates, proteins and lipids. These must be broken down to be absorbed into the body.

The chemical reactions required to break them down would be too slow without enzymes.

Enzymes are biological catalysts – they speed up chemical reactions.

Enzymes are required for most of the chemical reactions that occur in organisms. These reactions occur in the breakdown of chemical molecules, which we see in the digestive system.

Enzymes are also involved in the building up of chemical molecules elsewhere in the body.

Enzymes are proteins that have a complex 3D-shape. Each enzyme has a region called an active site.

The substrate – the molecule or molecules taking part in the chemical reaction – fits into the active site. Once bound to the active site, the chemical reaction takes place .

In an organism, the active site of each enzyme is a different shape. It is a perfect match to the shape of the substrate molecule, or molecules. This is essential to the enzyme being able to work. One enzyme is therefore specific to one substrate's chemical reaction, or type of chemical reaction.

This theory for the way in which enzymes work is called the lock and key theory.

Learn all about enzymes with Dr Alex Lathbridge.

Listen to the full series on ����ý Sounds.